More sophisticated production and new applications demand that adhesives deliver increased performance and stronger, more resistant bonds. For example, the adhesive may be required to adhere to poorly prepared, contaminated substrates without a primer. They may also be expected to display non-sag properties upon application to vertical surfaces, have rapid green strength development, and form bonds that can withstand elevated temperature exposure.
Polyurethane structural adhesives have been used for many years in industry to adhere substrates like fiberglass reinforced polyesters (FRP). U.S. Pat. No. 3,936,051 discloses typical commercially available polyurethane structural adhesives. The typical adhesive comprises an isocyanate functional component, e.g. a polyurethane prepolymer, and a curing agent. The prepolymer component is the reaction product of polypropylene glycol and toluene diisocyanate, and the curative is a blend of polypropylene ether glycol and N,N,N′,N′-tetrakis (2-hydroxypropyl) ethylene diamine. These adhesives are well known for their good flexibility and ability to adhere difficult substrates like FRP. They are also known for superior bonding of dissimilar substrates like FRP to cold rolled steel (CRS).
A common approach to improving adhesive properties is to focus on the curative component of the two component (2K) urethane adhesive system. The objective of many of these innovations is to impart non-sag properties to the applied adhesive. Non-sag adhesives can be applied to substrates in a vertical position without the adhesive dripping or sagging before the second substrate is placed over the adhesive.
U.S. Pat. No. 3,714,127 discloses the addition of di- and multi-primary amines to a 2K polyurethane-based adhesive to impart sag resistance.
U.S. Pat. No. 4,336,298 discloses the addition of para- or ortho-phenylene diamine to a curative blend to improve sag resistance of an adhesive system for bonding fiberglass reinforced polyester (FRP) panels.
U.S. Pat. No. 5,508,111 discloses the use of aryl or aromatic amines, like 1,8-napthalene diamine, in polyurethane adhesives for good sag properties.
U.S. Pat. No. 4,444,976 discloses secondary diamines can also be added to a curative for a 2K polyurethane adhesive to give non-sag properties.
U.S. Pat. No. 5,002,806 discloses a mixed curative containing a slow and fast reacting diamine for sag resistance in 2K structural primerless urethane adhesives.
Another approach is to add special additives to enhance the adhesive performance. U.S. Pat. No. 5,955,199 discloses the use of polyaldimine or polyketimine in the curative component of a 2K urethane structural adhesive.
U.S. Pat. No. 4,728,710 discloses the use of bicyclic amide acetals in an antifoaming 2K polyurethane adhesive.
U.S. Pat. No. 5,175,228 discloses a 2K polyurethane composition in which the prepolymer component contains a primary hydroxyl intermediate and has a sufficiently large excess of free isocyanate so that the ratio of free NCO groups in the prepolymer component to OH curative groups plus any amine groups in the curative component is from about 1.2 to about 2.0.
EP 0 279 566 discloses better adhesion to sheet molding compound (SMC) by incorporating a graft copolymer of poly(alkylene oxide) polyol and a vinyl monomer into a 2K polyurethane adhesive formulation.
U.S. Pat. Nos. 4,923,756 and 5,143,996 disclose the addition of small amounts of aliphatic polyisocyanates, like m-xylylene diisocyanate, promote primerless adhesion.
The current polyurethane structural adhesive patent art does not fully exploit the benefits of controlling the structure of polyurethane prepolymers. Current prepolymers specified in these patents are typically produced from the reaction of a polyisocyanate with a polyol in a 2:1 equivalents ratio. The manufacturer may change the type of polyisocyanate or polyol, but the ratio, and therefore the prepolymer distribution (2:1 adducts, 3:2 adducts, etc.) remains unoptimized. Instead, optimization often focuses, as illustrated by the previous patents, on the composition of the curative and different additives that are added to the formulation.
EP 0 827 995 discloses hot melt adhesives comprising a polyisocyanate prepolymer prepared by reacting a polyisocyanate with a functionality of at least 2 with a polyol with a functionality of at least 2, the reaction product comprising at least 90 wt % “perfect” prepolymer and less than 2 wt % unreacted isocyanate monomer and the prepolymer having a free NCO functionality ranging from 0.2 to 8 wt %.
A great deal of work has gone into improving two component polyurethane structural adhesive formulations through improvements in curative, primers, surface treatment and addition of various additives. However, tailoring of the chemical composition of the isocyanate functional component, or prepolymer, has yet to be fully explored as a means to improve the properties and performance of polyurethane structural adhesives. We have found unexpected advantages in using controlled isocyanate terminated prepolymers that contain controlled levels of residual isocyanate monomer and oligomer content.